When an electric motor is rotationally driven, as the rotational speed of the electric motor increases, the energizing timing is delayed due to the influence of armature reaction and the like. Thus, for example, a technology is disclosed in which an advance angle value in accordance with a speed-command voltage is derived and driving of an electric motor is controlled by using the derived advance angle value (hereinafter, “advance angle control”) (for example, Patent Literature 1 listed below).
In the advance angle control, it is typical to have a configuration in which information on an optimum-phase advance angle that is in accordance with the rotation speed of an electric motor is saved as a table and a microcomputer or a dedicated control IC controls the electric motor on the basis of the information on the optimum-phase advance angle characteristics stored in the table. The optimum-phase advance angle (also referred to as “optimum advance angle”) is an advanced phase angle at which operational efficiency of an electric motor becomes maximum. The “phase angle” described herein is a phase difference between the voltage induced by a stator winding (hereinafter, “induced voltage”) and the voltage applied to the stator winding from an inverter IC (hereinafter, “applied voltage”). When the applied voltage leads the induced voltage, the advance angle has a positive value.